What is Facebook's lifespan?

It's well documented that certain people have already left the social network or are planning to leave soon.

When does it become mainstream - and by that I mean 16-25 year olds are doing it - to deactivate your Facebook account? I don't think people will go social network-less... something a.) cooler and b.) better would have to take its place for a substantial amount of people to leave, and I don't think that product is out there at this point.